What Leonardo da Vinci concept is often credited as an early ball bearing?

Leonardo da Vinci is often credited with designing an early ball bearing.

His studies of friction included a mechanism in which balls moved between rings, reducing direct sliding contact. The idea appears in Leonardo’s notebooks from the late 15th or early 16th century and is frequently cited as an early example of rolling-element bearing design.

The historical claim needs care. Leonardo did not invent every form of bearing, and ancient and medieval engineers used other methods to reduce friction. Nor is it certain that his ball-bearing concept was manufactured and put into practical service. The surviving drawings show an important mechanical principle, but they do not establish a continuous path from Leonardo’s sketch to modern industrial bearings.

Modern ball bearings use precisely manufactured balls, races, cages, lubrication, and carefully controlled materials. Their purpose is similar to Leonardo’s concept: replacing sliding friction with rolling motion. A roller bearing uses cylindrical or tapered rollers instead of balls, so it is a related but different machine.